feat(client)!: use unique endpoint typestates

[dustypomerleau]

Thanks for all the time you've put into oauth2-rs!

I have been interacting with this crate via axum-login, and one challenge is the need to always remember the order of the typestate generics on Client. Updating the generics to use specific typestates for each endpoint type would be a breaking change, but it would make it possible to tell at a glance which endpoints are set without memorizing the order.

In short, it would change:

let client: BasicClient<
    EndpointSet,
    EndpointNotSet,
    EndpointNotSet,
    EndpointNotSet,
    EndpointSet,
> = todo!();

to:

let client: BasicClient<
    AuthEndpointSet,
    DeviceAuthEndpointNotSet,
    IntrospectionEndpointNotSet,
    RevocationEndpointNotSet,
    TokenEndpointSet,
> = todo!();